Oil cooling system for electric machines

Abstract

A system for cooling an electric motor (10), comprising: a lubrication circuit (20) capable of bringing coolant into contact with the working parts of the motor; a main tank (21) in the lower part of the motor housing; and a pump (22) for pumping coolant into the circulation circuit, the level (30) of the coolant in the housing interfering with the lower part of the rotor when the pump is stopped, and means (100) for controlling the pump are adapted to keep the pump stopped when the temperature of the coolant is below a given threshold and to command the start of the pump as soon as the temperature of the liquid reaches the threshold, thereby causing the liquid to be pumped by the main tank pump towards the secondary tank and reducing the level of the coolant in the housing.

Description

[Technical Field]

[0001] The present invention relates to a system for cooling an electric motor of a vehicle, and more precisely to a system for cooling and lubricating an electric machine used in a motor vehicle. [Background technology]

[0002] An electric machine typically has a stator and a rotor arranged coaxially, one inside the other. The rotor is formed from a rotor body carrying magnetic flux generators, such as permanent magnets or windings. The rotor is housed inside a stator, which also carries magnetic flux generators, typically in the form of windings, that generate a magnetic field that, in combination with the magnetic field generated by the rotor magnets or windings, enables the rotor to rotate.

[0003] Cooling of the windings of an electric machine plays a vital role since a significant part of the performance losses in an electric machine are accounted for by Joule losses which are proportional to the temperature of the winding wire.

[0004] Systems are known for cooling electric machines by circulating an oil-type coolant over their working parts, in particular their windings. For example, patent document WO 2018 / 206890 discloses such a system, which essentially comprises an oil circulation circuit that allows oil to come into contact with the working parts of the electric machine through jets that allow oil to be sprayed onto the working parts, a reservoir that can collect the oil that has cooled these working parts, a pump that allows oil to be re-injected from the reservoir into the circulation circuit, and a heat exchanger that allows the oil temperature to be kept below a temperature threshold. This mechanism makes it possible to ensure both the lubrication of the rotating components of the machine (dynamic seals and rolling bearings) and the cooling of the working parts of the machine (rotor and stator) to prevent the machine from overheating.

[0005] Such cooling systems are generally cumbersome, especially due to the presence of the pump, which is an electric pump rather than a mechanical pump, as it is necessary to provide maximum cooling of the machine at low speeds and high torque demands.

[0006] This pump is sized to the requirements of the system. In the case of sealed electrical machines, the seals and rolling bearings must be constantly lubricated. In other words, the pump must be kept in operation to ensure a minimum oil flow rate at all times for lubrication purposes, regardless of the conditions, and in particular the oil temperature. In cold weather, when the oil is in the negative temperature range and below a certain threshold, no cooling is required and the oil circulation ensured by the circuit pump only serves to lubricate the seals and rolling bearings of the machine.

[0007] On the other hand, pump sizing depends on the oil viscosity, which is very high at subzero temperatures and rapidly decreases to a low viscosity above a certain temperature threshold. In cold weather, high oil viscosity imposes significantly higher pressures on the circuit, which in turn requires higher torque demands from the pump, resulting in the need for costly oversizing of the pump.

[0008] Another problem is the noise generated by the pump, especially at low motor speeds, when maneuvering in traffic, for example in a parking lot or going uphill. Under these conditions, the noise generated by the pump is louder than the noise generated by the electric motor, which is clearly undesirable.

[0009] Furthermore, systems are known that cool the rotor by splash lubrication during operation, in which oil splashed by the rotor as it rotates cools the working parts of the machine and lubricates the seals and rolling bearings. However, in this case, the rotational speed of the machine is a limiting factor for proper operation of the machine. In particular, as the rotational speed increases, friction losses due to oil in the air gap and on the surface of the rotor increase significantly from a certain operating speed. In other words, this system allows the motor to operate efficiently only within a relatively limited range of rotational speed changes, without any degradation in the machine's performance. Summary of the Invention

[0010] Therefore, there is a need for a system for cooling electric machines, particularly electric traction machines in electric or hybrid vehicles, that is free from, at least in part, the above-mentioned constraints.

[0011] To this end, the invention relates to a system for cooling an electric machine, the electric machine having a rotor with a rotating shaft and a stator fixed to the inner wall of a casing of the machine and surrounding said rotor, the casing comprising at least one bearing housing supporting an end of said rotating shaft via a rolling bearing, the system comprising: - a lubrication circuit that allows a coolant to come into contact with the working parts of the electric machine; - a main reservoir located in the lower part of the casing and capable of holding a coolant; a pump connected to the main reservoir and capable of pumping the coolant through the lubrication circuit; In a system comprising: the amount of liquid in the main reservoir reaches a coolant level in the casing that interferes with the lower part of the rotor when the pump is stopped, the lubrication circuit leads to an auxiliary reservoir located in the upper part of the casing, the system comprises control means for controlling the pump at least as a function of the coolant temperature, the control means being designed, on the one hand, to keep the pump stopped when the coolant temperature is below a given threshold, so that the coolant is splashed into the casing by the splash lubrication of the rotor and is collected in a receptacle attached to the surface of the casing next to the bearing housing, which receptacle can guide the liquid to the bearing housing to lubricate and cool the rolling bearing, and, on the other hand, to command the start of the pump as soon as the coolant temperature reaches the threshold, so that liquid is drawn by the pump from the main reservoir into the lubrication circuit and towards the auxiliary reservoir, thereby lowering the coolant level in the casing.

[0012] This allows the cooling of the rotor and stator and the lubrication of the machine to be ensured without the use of a pump, thanks to this mechanism designed for cold weather operation, where the oil viscosity is high, when the temperature of the coolant, usually oil, is below a given threshold, e.g., 0°C. This rotor splash lubrication allows the oil to be splashed into the casing, in particular into a receptacle designed to guide the oil towards the bearing housing. Therefore, the pump does not need to be sized to meet high oil viscosity requirements, since it is not used during cold weather operating conditions where lubrication or cooling is required. This reduces the power and maximum torque required by the pump, which is only used at oil temperatures above the given threshold, which accompany low oil viscosity.

[0013] As soon as the oil temperature reaches a certain threshold, primarily for positive temperatures, the pump is commanded to start, thereby directing oil into the lubrication circuit, thereby ensuring that the secondary reservoir is filled while simultaneously lowering the oil level in the casing, thereby preventing contact between the lower part of the rotor and the oil. This reduced contact is significantly favorable for the operation of the electric machine at high rotational speeds, since it significantly reduces the reduction in friction load due to the oil. In this case, cooling and lubrication are essentially ensured by lubricating oil by a pump into the lubrication circuit, which is provided for spraying onto the working parts of the machine. In contrast to a cooling and lubrication mode that relies solely on rotor splash lubrication, this cooling mode is not limited by the motor's rotational speed, which makes it possible to increase the operating speed of the machine without affecting its performance.

[0014] In other words, the choice made possible by the system of the present invention, as a function of oil temperature, between a cooling and lubricating mode with the pump at rest, solely by splash lubrication of the rotor in oil, and a cooling and lubricating mode with the pump running, essentially by injection of oil via the lubrication circuit and the secondary reservoir onto the machine parts to be cooled and lubricated, advantageously makes it possible to achieve an optimal size of the pump, mainly at positive temperatures, while increasing the operating speed of the machine without affecting its performance.

[0015] Advantageously, said sub-reservoir has at least one discharge located at the outer edge of the stator and provided with a variable flow nozzle capable of spraying the cooling liquid onto a portion of the outer longitudinal surface of said stator.

[0016] Advantageously, the sub-reservoir has two discharge ports located opposite each other, or one discharge port for spraying the central outer edge portion of the stator.

[0017] Advantageously, the lubrication circuit includes variable flow nozzles arranged in the casing and in the bearing housing for spraying the rotor winding overhangs.

[0018] Advantageously, said control means are designed to control the flow rate of the pump as a function of the oil level in the casing.

[0019] Advantageously, said control means are designed to control the flow rate of the pump as a function of the temperature of the working part of the machine.

[0020] Advantageously, said control means are designed to control the pump flow rate as a function of the lubrication requirements of the machine.

[0021] Advantageously, the control means are designed to at least reduce the speed of the pump, or even stop the pump, when the noise emitted by the pump is greater than the noise emitted by the electric machine.

[0022] Advantageously, said socket is designed to fit into the upper corner of the bearing housing in which it is located and to communicate, through a duct passing through this upper corner of the bearing housing, with the space located between the rolling bearing and the dynamic seal, which extends the wall of the casing to seal the machine at the outlet of the rotating shaft.

[0023] The invention also relates to an electric or hybrid motor vehicle comprising an electric traction machine and a cooling system as described above for cooling said machine.

[0024] Many further characteristics and advantages of the invention will become clearly apparent from the following description of the invention, given entirely as a non-limiting indication, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0025] [Figure 1] 1 is a schematic longitudinal section through an electric machine cooled by a cooling system according to the invention; [Figure 2] FIG. 4 is a schematic diagram of the operation of the control means associated with the pump. DETAILED DESCRIPTION OF THE INVENTION

[0026] According to the embodiment of the invention shown in FIG. 1 , a cooling system according to the invention is provided for cooling an electric machine 10, in particular an electric traction machine for an electric or hybrid vehicle. The electric machine 10 has a casing 11 in which a stator 12 is fixed, which consists mainly of magnetic soft iron laminations and copper windings inserted into slots in the soft iron laminations. A rotor 13 is housed within the stator 12, which, according to this embodiment of the invention, consists of magnetic soft iron laminations forming protruding magnetic poles around which copper windings 14 are wound. The rotor 13 is mounted on a rotating shaft 15, which is fixed at one end to the rotor 13 and at the other end to the casing of the electric machine. Both ends of the rotating shaft 15 are held by rolling bearings 16, in particular ball bearings, housed in substantially cylindrical bearing housings 17 attached to the rotating shaft and formed in the vertical wall of the casing 11. Specifically, ball bearings 16 are housed in the cylindrical inner portion of the bearing housing. Dynamic seals 18 hermetically seal the shaft ends of the electric machine at the ball bearings 16.

[0027] The cooling system according to the invention is used to cool the working parts of the electric machine 10, these being the stator 12 and the rotor 13, and also to lubricate and cool the seals and rolling bearings.

[0028] To this end, the cooling system comprises, inter alia, a lubrication circuit 20 which makes it possible to bring a coolant, preferably oil, into contact with the components of the electric machine 10 by injecting it, the oil being fed to the lubrication circuit 20 by means of a pump 22 from a main reservoir 21 situated below the machine in the lower part of the casing. A system comprising a strainer 23 and pipes conducts the oil from the reservoir 21 to a pump 22, which is itself connected by a pipe conducting the pumped oil to a heat exchanger 24 which cools the oil received from the pump 22 before re-injecting it into the lubrication circuit 20, which conducts the oil cooled in the heat exchanger 24 and makes it possible to inject it directly onto the heat-generating components of the electric machine. The oil thus injected recovers the thermal energy generated by the machine and then returns to the reservoir 21 by gravity.

[0029] For this purpose, the lubrication circuit has a line at the outlet of the exchanger 24 that leads the cooled oil to an inlet 25 located in the bottom of the casing of the electric machine 10, which forms the vertical wall of the casing 11 of the electric machine 10. This inlet 25 leads the oil cooled by the exchanger to a channel 26 in the bottom of the casing, which leads, on one side, to a nozzle located opposite the rolling bearing 16 of the rotating shaft 15 of the electric machine 10, and, on the other side, to a nozzle 26a located opposite the rotor winding extension 14a. These nozzles are preferably variable-flow nozzles. The inlet 25 also leads the oil cooled by the exchanger to upper inlets 41 and 42 of a secondary reservoir 27 located at the top of the electric machine, in the upper part of the casing. This secondary reservoir 27 extends to the outer edge of the stator, essentially over its entire length. The sub-reservoir is adapted to be filled with oil and has in its lower part a central discharge 28 arranged at the outer edge of the stator and provided with a nozzle, preferably a variable flow nozzle, which makes it possible to spray oil onto the central part of the outer longitudinal surface of the stator 12. In a variant, the reservoir 27 may comprise two discharges arranged opposite each other, preferably substantially at opposite longitudinal ends of the sub-reservoir 27. Preferably, the reservoir 27 has an additional overflow outlet 43 which prevents the reservoir from filling up completely.

[0030] Oil from the cooling nozzles arranged facing the rotor winding extensions 14a and the outer edge of the stator cools the stator and rotor, then falls by gravity into the main oil reservoir 21. At the same time, oil from the lubrication nozzles arranged facing the rolling bearings 16 also falls by gravity into the main oil reservoir 21. The flow rate of the lubrication nozzles is preferably adjusted to be less than the flow rate of the cooling nozzles.

[0031] Furthermore, according to the invention, the amount of liquid in the main reservoir 21 is adapted so that, when the pump is stopped, the oil level in the casing reaches a level 30 which interferes with the lower part of the rotor. As shown in Figure 1, the oil level 30 in the lower part of the electric machine 10 is at the level of the lower windings of the rotor 13. The lower part of the rotor is therefore immersed in oil.

[0032] Advantageously, the temperature of the oil is measured by a temperature sensor 31 located in the main reservoir 21 .

[0033] A first cooling mode, called cold-weather cooling mode, is now described when the oil temperature measured by sensor 31 is below a predetermined temperature threshold. This threshold advantageously allows for defining the temperature at which the oil viscosity level reaches such a level that a significant torque is required from the pump to lubricate the oil in the circuit. This threshold is preferably set, for example, to 0°C, since oil viscosity is extremely high at negative temperatures. To avoid the resulting need to oversize the pump, the pump is commanded to shut down when the oil temperature measured by sensor 31 is below the predetermined temperature threshold. As shown in FIG. 1, the oil level in the casing reaches level 30, thereby immersing the lower part of the rotor in oil. Therefore, when the machine is running, the rotation of the rotor coupled to the rotating shaft causes oil to be splashed into the casing by the rotor's splash lubrication. More precisely, the rotational movement of the oil-immersed rotor directs oil over the entire cylindrical surface of the rotor, which allows the rotor windings to cool. Furthermore, oil is splashed by the rotor onto the cylindrical inner surface of the stator, thereby cooling the stator windings. A portion of the splashed oil is also collected in a receptacle 32 attached to the casing surface beside the bearing housing 17. This receptacle is provided to guide the collected oil into the bearing housing, thereby lubricating and cooling the seals and rolling bearings mounted therein. The receptacle is fastened or screwed to the casing surface beside the bearing housing, for example. More precisely, the receptacle 32 is designed to fit into the upper corner of the bearing housing in which it is located and to communicate, through a duct penetrating this upper corner of the bearing housing, with the space located between the rolling bearing and the dynamic seal, which extends the casing wall to seal the machine at the exit point of the rotating shaft. In this way, the oil efficiently lubricates the ball bearings and the dynamic seal.Cutouts are further provided to prevent oil flowing between the dynamic seal and the ball bearing from completely filling the seal housing and to allow the oil to drain into the lower portion of the casing.

[0034] Thus, in this cold weather cooling mode, the working parts of the machine are cooled only by the oil that is thrown off by the rotor as it rotates, and the rolling bearings of the machine are also lubricated by the oil that is thrown off by the rotor during operation. The pump 22 of the oil lubrication circuit 20 is then at a standstill and is therefore not used in this cold weather cooling mode, where the viscosity of the oil is high, which advantageously allows the pump to be smaller in size thanks to the reduced power required from it, which means a lower cost pump, which is a competitive advantage of the electric machine cooled by the system of the invention.

[0035] In contrast, as soon as the oil temperature reaches a predetermined threshold, for example 0°C, at which the oil viscosity rapidly converges to a low value, a second cooling mode, called high-temperature cooling mode, is implemented and the pump 22 is started. The start-up of the pump 22 is therefore advantageously controlled by the oil temperature measured by the temperature sensor 31. When the pump 22 is started, oil is guided by it through the strainer 23 and the heat exchanger 24 into the lubrication circuit 20, one part of which is then guided into a channel 26 provided in the bottom part of the casing to spray onto the rotor and the rolling bearings, and another part of which is guided into an auxiliary reservoir 27 located in the upper part of the casing to fill it.

[0036] As the pump 22 starts and the secondary reservoir 27 fills, the oil level 30 in the casing drops until it reaches the new reference oil level 40 in FIG. 1, thereby preventing oil from coming into contact with the rotor. This advantageously reduces mechanical friction losses due to the resistive torque generated by the oil in contact with the rotor and in the air gap, particularly as the machine's operating speed increases, thereby improving machine performance at oil temperatures above a certain threshold. The pump's ability to operate at oil temperatures above this threshold allows for increased speeds, particularly above 14,000 rpm, up to 16,000-20,000 rpm, without performance degradation.

[0037] The pump 22 can be optimally controlled as a function of several criteria. With reference to Figure 2, control means 100 for controlling the pump 22 are shown. The control means 100 are designed, as explained above, to command the stop / start of the pump as a function of the received oil temperature measurement Toil. The pump control means 100 are also designed to control the flow rate of the pump 22 as a function of the temperature of the working parts of the machine and as a function of the cooling requirements of the machine, which are set taking into account the oil temperature. The temperatures of the working parts of the machine are given, for example, by an estimate of the rotor temperature Trotor and a measurement of the stator temperature Tstator, which are obtained by a temperature sensor 33 installed in the stator.

[0038] The control means may also be designed to control the pump flow rate as a function of the oil level in the casing, in this regard the control means 100 is set to ensure a minimum pump flow rate in order to drain the main oil reservoir to obtain an oil level suitable to prevent oil-rotor contact.

[0039] The control means may also be designed to control the flow rate of the pump as a function of the lubrication requirements of the machine, which is set, for example, on the basis of a map integrated in the control means, which map makes it possible to guarantee a minimum flow rate supplied by the pump for lubrication purposes.

[0040] If the noise emitted by the pump is greater than the noise emitted by the machine, the control means 100 can further be designed to reduce the pump speed or even stop the pump. To objectively quantify the noise emitted by a running machine or pump, various methods of noise evaluation can be used, for example by analyzing the generated acoustic power. If the pump is stopped or its speed is restricted when the operating conditions require lubrication or cooling of the machine, the machine is usually at risk of reaching its thermal limit and / or the seals or rolling bearings are at risk of deterioration. However, in the system according to the invention, even if the pump is stopped or restricted so that its noise is lower than the noise of the machine, any lubrication or cooling failure is advantageously prevented, because in that case the secondary oil reservoir is drained to cool the machine, thereby increasing the oil level in the casing again to level 30 shown in FIG. 1 and immersing the rotor in oil, thereby ensuring that the rotor's splash lubrication cooling works properly.

Claims

1. A system for cooling an electric machine (10), said electric machine having a rotor (13) with a rotating shaft (15) and a stator (12) fixed to the inner wall of a casing (11) of said machine and surrounding said rotor, said casing (11) comprising at least one bearing housing (17) supporting an end of said rotating shaft via a rolling bearing (16), a lubrication circuit (20) allowing a coolant to come into contact with the working parts of said electric machine; a main reservoir (21) located in the lower part of said casing and capable of holding said cooling liquid; a pump (22) connected to said main reservoir and capable of pumping said coolant into said lubrication circuit; In a system comprising: The amount of liquid in the main reservoir reaches a coolant level (30) that, in a stopped state of the pump, interferes with the lower part of the rotor in the casing, the lubrication circuit leads to a secondary reservoir (27) located in the upper part of the casing, the system comprises control means (100) for controlling the pump at least as a function of the temperature (Toil) of the coolant, the control means on the one hand keeping the pump (22) stopped when the temperature of the coolant is below a given threshold, so that the coolant is splashed into the casing by splash lubrication of the rotor, and the base the liquid is collected in a receptacle (32) attached to the face of the casing beside the bearing housing, the receptacle being designed to be able to guide the liquid into the bearing housing to lubricate and cool the rolling bearings, and on the other hand to command the start of the pump (22) as soon as the temperature of the cooling liquid reaches the threshold value, so that the liquid is drawn by the pump from the main reservoir into the lubrication circuit and further towards the secondary reservoir, thereby lowering the level of cooling liquid in the casing.

2. 2. The cooling system according to claim 1, wherein the sub-reservoir (27) has at least one discharge part (28, 29) arranged at the outer edge of the stator and provided with a variable flow nozzle capable of spraying the cooling liquid onto a portion of the outer longitudinal surface of the stator.

3. 3. The system of claim 2, wherein the sub-reservoir has two discharge portions located opposite each other or one discharge portion for spraying the central outer edge portion of the stator.

4. 4. A system according to any one of claims 1 to 3, characterized in that the lubrication circuit (20) has variable flow nozzles arranged in the casing and the bearing housing for spraying the rotor winding overhangs.

5. 5. A system according to any one of claims 1 to 4, characterized in that the control means (100) are designed to control the flow rate of the pump (22) as a function of the oil level (30) in the casing.

6. 6. A system according to any one of claims 1 to 5, characterized in that the control means (100) are designed to control the flow rate of the pump (22) as a function of the temperature (Tstator, Trotor) of the working part of the machine.

7. 7. A system according to any one of claims 1 to 6, characterized in that the control means (100) are designed to control the flow rate of the pump (22) as a function of the lubrication requirements of the machine.

8. 8. A system according to any one of claims 1 to 7, characterized in that the control means (100) are designed to at least reduce the speed of the pump and even stop the pump when the noise emitted by the pump is greater than the noise emitted by the electric machine.

9. 9. A system according to claim 1, wherein the socket (32) is designed to fit into an upper corner of the bearing housing (17) in which it is located and to communicate with the space located between the rolling bearing (16) and a dynamic seal (18) through a duct passing through the upper corner of the bearing housing, the dynamic seal extending the wall of the casing to seal the machine at the exit point of the rotating shaft (15).

10. An electric or hybrid motor vehicle comprising an electric traction machine and a cooling system for cooling said machine according to any one of claims 1 to 9.

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